I recently bought a used craftsman self propelled snowblower. When I went to pick it up I made sure everything was working fine, all the gears worked, started fine, etc. I was having a problem loading it into my truck because it was so tall, so I took the wheels off it (just pulled the two pins on the ends). I took it home, put the wheels on it, started it up, and couldn't get it to move. I took the plate on the bottom off to see if there was a chain or something that might have slipped, but it only looks like there are sprockets. (here is the link to the diagram)

http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Snow-removal-equipment-Parts/Model-247886640/0247/1507200/P0102095/00006?blt=06&prst=&shdMod=

When I engage the traction plate, the gears spin, but there doesn't look like there is anything on the axle to cause it to spin. If I pick up the snowblower, I can spin the wheels with my hands. There appears to be a groove in the inside of the gear, and a groove on the outside of the axle. When I look at the diagram there is something called a HI-PRO key; which I don't seem to have.

Is this little key the missing piece that gives all the power to turn the axle? It's quite possible when i removed the wheels that it fell out if it's only held in place by friction. It's a $2.55 part, but has to be ordered. I've seen many posts about the friction plate and belts being a cause of self propelleds not moving; but those all seem fine.

I would have expected to see it more often in posts if it's something as simple as that, but I can't find anything about it. Maybe I'm not using the correct terms. Thanks for the help!
Nick